The move, which will be completed in January, is subject to the agreement of personal terms and a medical.
Atletico said the 28-year-old, who left the club to join Chelsea in 2014, would have a medical in the next few days.
Spain international Costa has not played for the Blues this season, and spent much of August in his native Brazil.
Chelsea play at Atletico next Wednesday in the Champions League. SHARE THIS STORY USING ANY OF THE BUTTON BELOW ⬇ PLACE YOUR TEXT ADVERT BELOW ⬇⬇⬇
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